1. Field of the Invention
The present invention relates generally to electromagnetic heating devices and, more specifically, to improved induction heating devices and methods employing same.
2. Description of the Related Art
There currently exists a wide number and variety of induction heating devices. One such induction heating device is described in U.S. Pat. No. 4,521,659, issued to Buckley et al. on Jun. 4, 1985. The heating gun described therein uses a tank circuit which includes a capacitor and an inductor coil. The inductor coil is wrapped around a U-shaped pole piece which has a gap formed between the ends thereof. The capacitor of the tank circuit is connected to a power supply which provides an alternating current of predetermined frequency.
The aforementioned patent describes a technique of joining two sheets of material by placing a susceptor, such as a wire screen, between the sheets with adhesive therebetween. The heating gun is positioned above the two sheets and the screen with the ends of the pole piece touching one of the sheets and with a gap of the pole piece located above the area where the sheets are to be joined. An alternating current from the power source enters the tank circuit and the capacitor increases the Q of the tank circuit, in turn increasing the current flow through the inductor coil. Direction of the current along the inductor coil reverses at each cycle of alternating current. The current flow in the inductor coil creates a magnetic flux within the turns of the coil. The flux is picked up by the pole piece and carried to either of the two ends. The flux then jumps to the susceptor rather than across the gap to the other of the two ends. Since the sheets of material are transparent to magnetic flux, the flux is easily transferred to the screen through the sheets. The rapidly changing direction of current in the coil causes the flux to change constantly in magnitude and direction. This is also true in the metal screen as well as in the pole piece. Hysteresis creates eddy currents in the screen which result in heat being generated in the screen.
In the past, electric heaters have been used to heat a bonded joint between two objects while applying tension thereto in order to determine the tensile strength of the bond. The primary disadvantage of the prior art is that the amount of time and power required to heat the materials is rather large. When testing materials, electric heaters, usually requiring kilowatts of power, require one half hour or more to reach the test temperature. Heat-to-shrink metal couplings and fittings have been used for flightline, depot, and battlefield repair of military aircraft hydraulic tubing. Military use of the couplings and fittings require a rapid and reliable heating device that is lightweight, portable and easy to use. A suitable heating device must provide a focused heat zone which generates elevated temperatures in a relatively short period of time and can maintain a predetermined temperature for a controlled period of time, if necessary. The desirability for all weather use limits the applicability of heating devices which require extensive fuel lines, wiring harnesses, power sources, etc. Moreover, aerospace applications would require the device to function efficiently and reliability in the hard vacuum of space.
Prior heating devices include open flame, forced hot air, chemical heat, and resistive heater devices. All of these techniques and devices suffer from one or more disadvantages, most significantly that of the potential for explosion when working around hazardous, inflammable fuels. Moreover, the aforementioned devices and techniques are difficult for a technician or a mechanic to handle with one hand, and are generally not capable of efficiently and effectively producing a uniform heat zone circumferentially around a cylindrical shrink fit coupling.
The aforementioned prior art heating devices and methods are also of limited usefulness in other areas, such as the joining of band saw blade ends, the soldering of thermocouple wires, and the heating of test materials to determine bond region separation strength.
Resistance blade brazing and annealing methods, for band saw blades, typically employ high power consumption, heavy weight and slow heating devices which result in high operating costs, non-portability, and deterioration of blade tensile strength. In addition, resistance brazing uses the blade as part of the electrical circuit which requires that the blade ends touch at the joint in order to provide circuit continuity and thus heat. Since metal expands when heating, buckling of the blade joint often occurs and a short blade life results. Open flame blade brazing and annealing devices produce combustion hazards, provide no control of heating cycle, and require trained and experienced personnel to operate. Moreover, open flame brazing normally results in blade scale formation, deformation of the blade joint and deterioration of the blade tensile strength. Heating of the blade during any blade joining process causes a hardening of the heated portion of the blade, thereby reducing blade flexibility and thus blade life.
With respect to annealing blade joints, proper annealing of the blade joint is required to soften and render the blade less brittle. Blade joint annealing is of particular concern to blade manufacturers who are seeking an economic and reliable means of providing this function.
With respect to thermocouples, a thermocouple is comprised of two wires of dissimilar metals joined at one end. The open circuit voltage is dependent on the junction temperature and the composition of the two metals. A thermocouple can be used over a range of temperatures and optimized for various atmospheres. Thermocouples are the most rugged temperature sensors available and can be fabricated by soldering or welding. In the past, electric welders and torches have been used to join the leads of the thermocouples. However, oxidation and fixturing are major drawbacks associated with the prior art.
With respect to materials testing, the heating devices and methods mentioned above have been used, but also suffer from various drawbacks. For example, the amount of time required to heat the materials is long and the power requirements are high when using electric heaters. About a half an hour or more is required to reach the test temperature. The time and power requirements, are major disadvantages when attempting to produce thermocouples on a commercially viable scale, or in the field for small repair operations.